Metal Injection Moulding (MIM) is an interesting technique for producing high density sintered components of complex shapes. In general fine carbonyl iron powders are used in this process. Other types of powders used are gas atomized and water atomized of very fine particle size. However, the cost of these fine powders is relatively high. In order to improve the competitiveness of the MIM process it is desirable to reduce the cost of the powder used. One way of achieving this, is by utilizing coarser powders. However, coarse powders have a lower surface energy than fine powders and are thus much less active during sintering. Another issue is that coarser and irregular powders have a lower packing density and thus the maximal powder content of the feedstock is limited. A lower powder content results in a higher shrinkage during sintering and may lead to inter alia in high dimensional scatter between components produced in a production run.
Literature suggests reducing the amount of carbonyl iron by adding certain amount of coarser iron powder and optimizing the mixing ratio, in order not to lose too much sinterability and pack density. Another way to increase sinterability is by adding ferrite phase stabilizers such as Mo, W, Si, Cr and P. Additions of 2-6% Mo, 2-4% Si or up to 1% P to mixes of atomized and carbonyl iron have been mentioned in literature.
U.S. Pat. No. 5,993,507 discloses blended coarse and fine powders compositions containing silicon and molybdenum. The composition comprises up to about 50% coarse powder and the Mo+Si—content varies from 3-5%.
U.S. Pat. No. 5,091,022 discloses a method of manufacturing a sintered Fe—P powdered metal product having high magnetic permeability and excellent soft magnetic characteristics, using injection molding with carbonyl iron below 5 μm.
U.S. Pat. No. 5,918,293 discloses an iron based powder for compacting and sintering containing Mo and P.
Normally the solid loading (i.e. the portion of iron-based powder) of an iron-based MIM feedstock (i.e. the iron-based powder mixed with organic binder ready to be injected) is about 50% by volume which means that in order to reach high density after sintering (above 93% of theoretical density) the green component must shrink almost by 50% by volume, in contrast to PM components produced through uniaxial compaction which already in green state obtain relatively high density. Therefore fine powders having high sintering activity are normally used in MIM. By elevating the sintering temperature coarser powders may be used, a drawback however with using elevated sintering temperatures is that grain coarsening may be obtained and hence lower impact strength. The present invention provides a solution for this problem.
It has unexpectedly been found that a feedstock comprising coarse iron-based atomized powder composition according to the invention, with a relatively low total amount of ferrite stabilizers, can be used for powder injection molding in order to obtain components with a sintered density of at least 93% of the theoretical density. Further, it has been noticed that apart from obtaining components having a sintered density above 93%, a surprisingly high toughness, impact strength, can be obtained if the powder contains a specified amount of molybdenum and phosphorous and have a certain metallographic structure.